The present invention relates to acoustic deterrence, and in particular to an acoustic deterrent device and methods for deterring mammals, most particularly but not exclusively marine mammals.
Worldwide, farming of marine and diadromous finfish species has experienced tremendous growth rates, showing a ten fold increase over the last three decades. This increase in potential food resources presented in a marine environment has brought about increased interactions with predatory species. One common group of predators is marine mammals who exploit food resources depending on their profitability and potential costs, which include dive depths as a major factor. The shallow depth of fish farms thus makes them particularly attractive to predators.
In particular, predatory behaviour of pinnipeds is a major concern, causing a variety of economical and market related risks for the fish farm owner. Accordingly, there is much interest in developing anti-predator control methods.
These methods include net modifications, lethal or non-lethal removals, population control and aversive conditioning. However, each of these methods has their own drawbacks. For example, the addition of a second net can cause tangling of predators and non-predatory species; and lethal removals as well as population control may have an impact on populations and raise ethical concerns over the treatment of the animals. Culling of higher order predators can also have negative impact on predation rates by other predators, for example, pinnipeds forage on predatory fish species around the net pen which in turn potentially feed on aquaculturally important species. Also, emetic aversion conditioning requires that individuals learn to associate treated fish with sickness, and this can be hard to achieve when predator numbers are high.
One anti-predator control method that avoids these pitfalls is the use of acoustic deterrent devices (ADDs). These have traditionally been considered to be a benign solution. However, they do present certain problems with respect to the effects they have on other marine wildlife and with habituation, where a target species motivated by a food source ceases to be deterred by the acoustic signals.
The acoustic power, also referred to as source level, of the ADDs can cause temporary or permanent hearing damage both to the targeted species and to other wildlife, and the noise pollution is in general an environmental hazard. Further, both targeted and non-targeted species can be excluded from their natural habitat within a wide radius of the fish farms. These concerns have led some governments to restrict or even ban the use of ADDs.
A further problem is the habituation of the target species to the sound. In extreme cases, the sound which is intended to be aversive acts as a “dinner bell” and actually serves to attract predators, rather than deter them. Also, if predator sounds are used as an aversive noise, habituation is dangerous for the target species once they had returned to their normal habitat.
Also, existing power levels and signal cycles impose heavy duty cycles on the batteries used as power sources in the transducer units.
Accordingly, it would be desirable to provide an acoustic deterrent device that is highly effective, but does not damage the environment is species specific and avoids habituation.